Question

What is the maximum concentration of equimolar solutions of ferrous sulphate and sodium sulphide so that when mixed in equal volumes, there is no precipitation of iron sulphide? (For iron sulphide, $K_{sp}$ = 6.3 x 10-18⁻¹⁸)

Answer 1

Let's assume maximum concentration of each solution is x mol/L.

[FeSO4] = [Na2S] = x/2

So,

[Fe2+] = [FeSO4] = x/2

We also know from the reaction that, [S2-] = [Na2S] = x/2

FeS → Fe2+ + S2-

Therefore, Ksp = [Fe2+][ S2-]

So, 6.3X 10–18 = x/2 X x/2 = x2/4

So, x = 5.02 × 10–9 M.

Answer 2

Answer:

Explanation:

Let's assume maximum concentration of each solution is x mol/L.

[FeSO4] = [Na2S] = x/2

So,

[Fe2+] = [FeSO4] = x/2

We also know from the reaction that, [S2-] = [Na2S] = x/2

FeS → Fe2+ + S2-

Therefore, Ksp = [Fe2+][ S2-]

So, 6.3X 10–18 = x/2 X x/2 = x2/4

So, x = 5.02 × 10–9 M.